We know from the pictographic history that the people in ancient Egypt were fond of beer and bread. Fortunately, they sent samples of these food items along with some of their more esteemed citizens to sustain them in the afterlife. Also, samples were available from rubbish heaps of ancient villages, allowing for the comparison of funerary goods with everyday foods. Additional luck was that the dry conditions have kept some samples from deteriorating even after thousands of years. Whereas they did leave us samples, they didn't leave recipes. There have been many theories about how they brewed and baked in ancient Egypt, but proof was lacking. Recently, Delwen Samuel of Cambridge University used optical and scanning electron microscopy (SEM) to determine how the ancient Egyptians brewed beer and baked bread.

Over the years, efficiency of backscattered electron detectors has improved. Robinson has developed a unique system whereby two BSE detectors are used, offering an array of benefits to the SEM operator. The system consists of two detectors with scintillators of dedicated geometrical shape, providing a collection area greater than 2000 mm2. They are situated against the axis of the electron beam, with the right and left signals combining to form the overall image signal - one that is 1.5 times greater than that of a single detector. The omnidirectional image illumination, similar to that attained thorough a traditional SE detector, prevents shadowing due to the 180° collection angle.

Although the benefits of operating the scanning electron microscope at low beam energies have been evident since the earliest days of the instrument, the successful utilization of the SEM under these conditions has required the development of high brightness field emission electron source, advanced lenses, and clean vacuums. As these technologies became available the level at which imaging became regarded as “low energy” has fallen from 10 keV, first to 5 keV, and more recently to 1 keV. At this energy state of the art, instruments can now provide an excellent balance between resolution - which becomes worse with decreasing energy - and desirable goals such as the minimization of sample charging and the reduction of macroscopic radiation damage - which tend to become more challenging as the energy is increased.

The first in a series of articles looking at developments in X-ray detectors and Field effect transistors. In recent years we have seen both Si(Li) and Intrinsic Silicon Detectors with resolutions below 130 eV. These same detectors are capable of measuring the soft X-rays like Be K, B K, C K etc. with good resolution and with peak positions reasonably close to the exact position for these lines. The Carbon peak is the one most affected by the L absorption edges in Silicon and in some circumstances can be shifted as much as 30 eV down in energy. Shifts can also result from the settings of amplifier threshold discriminators and ADC designs. At higher energies (e.g. Mo K) the Si(Li) detector, if not properly drifted can also have a down shift by as much as 200 eV.

I was one told by a Nobel Laureate in Chemistry that light microscopy was simply a service foundation. By this he meant to class the microscope with computers, gas chromatographs, infrared spectrophotometers, x-ray diffractometers, mass spectrometers, etc. With all due respect to this gentleman and to these other instruments, there is a vital difference between the polarized light microscopes (PLM) and each of these instruments. First, a trained microscopist requires far more training than a qualified operator of, and interpreter of data from these other instruments. Second, there is considerably more basic physical and chemical information observable and measurable with PLM.

In my last article, I discussed the need for scheduled system checks on scanning electron microscopes. The article used an experience at a local SEM laboratory to point out the importance of monitoring your instrument by performing simple but regular system performance evaluations.

In this article I will discuss some principles behind the performance of imaging systems such as scanning electron and scanning probe microscopes. We will talk about the concepts of image linearity and dimensional calibration.